Preheat oven to 350°F (175°C). Lightly grease a mini muffin pan.
Make the crust: In a bowl, cream butter and powdered sugar together until fluffy. Add salt and flour; mix until dough forms.
Press dough into the bottoms and up the sides of each mini muffin cup to form small shells.
Bake crusts for 10–12 minutes or until just lightly golden. Remove and cool slightly.
Make lemon filling: In a medium bowl, whisk together eggs, sugar, lemon juice, zest, and flour until smooth.
Pour filling into the baked tart shells until just below the rim.
Bake again for 10–12 minutes, or until the centers are just set.
Cool in pan for 10 minutes, then gently remove to a rack. Chill for 1 hour before serving.